The results suggest physicians should carefully monitor the bone health of patients placed on the medication and that their patients should take steps to decrease the risk of osteoporosis.

Warfarin is often given to patients with atrial fibrillation, irregular contractions of the upper chambers of the heart. By interfering with vitamin K's role in clotting, the drug decreases formation of blood clots, which often accompany atrial fibrillation. But because vitamin K also interacts with osteocalcin - a protein vital for bone formation - warfarin's antagonism of vitamin K has the potential to affect bone strength as well.

"We did a retrospective study of Medicare records for about 15,000 patients hospitalized with atrial fibrillation, and we identified fractures related to osteoporosis," says Brian Gage, M.D., associate professor of medicine and medical director of Barnes-Jewish Hospital's Blood Thinner Clinic. "Our analysis showed that long-term use of warfarin - longer than one year - led to a 25 percent increase in the incidence of fracture."

The study included about an equal number of men and women with an average age of 80. In the general population, 80 percent of those affected by osteoporosis are women, and women in this study were more likely to have an osteoporotic fracture than were men. But the women's risk of fracture did not increase by a statistically meaningful amount on long-term warfarin therapy. However, men in the study who took warfarin for more than a year had a 63 percent higher incidence of fracture than men who did not take the blood thinner.

Patients taking warfarin for less than one year did not have increased fracture risk, and patients taking beta-blockers had fewer fractures than patients not taking beta-blockers.